A transducer may be broadly defined as a device which converts one form of input energy to another form of output energy. A typical microelectronic capacitive transducer includes a pair of electrically conductive plates arranged in spaced apart relation. When configured to convert electrical energy into mechanical energy, the transducer operation is based upon the principle of electrostatic attraction caused by electrically charging the opposing plates. For example, when electrical energy is input to the transducer in the form of a voltage applied between the plates, the plates are drawn together by electrostatic attraction. If the plates are free to move together, the input electrical energy is converted into mechanical energy.
The plates of an electronic transducer may also be used to generate an electrical signal from an input of mechanical energy. For example, the plates may first be charged by an applied electrical voltage. The plates may then be disconnected from the charging source and mechanical energy used to move the plates either closer together or farther apart. As the distance between the plates is changed, the voltage between the plates changes thereby converting the mechanical energy into an electrical signal.
Accordingly, a microelectronic capacitive transducer may be used as an actuator or a sensor. As an actuator, the transducer may convert electrical energy into mechanical motion. As a pressure sensor, the transducer may convert mechanical motion into an electrical signal responsive to pressure changes.
Microelectronic capacitive transducers or sensors are known which generate an electrical signal responsive to a pressure difference between two fluids. If at least one of the two plates is a diaphragm and may deflect or otherwise move, a difference in the pressure exerted on the two sides of the diaphragm will cause it to move relative to the other plate as disclosed, for example, in an article by Clark and Wise entitled "Pressure Sensitivity in Anisotropically Etched Thin-Diaphragm Pressure Sensors," IEEE Transactions on Electron Devices, Vol. ED-26, No. 12, December 1979. The article discloses a differential capacitive pressure sensor including a metallized thin silicon diaphragm and a second plate provided by a silicon support chip. A reference cavity separates the two plates, and this cavity has an inlet to allow access to a first fluid providing a reference pressure. The face of the silicon diaphragm opposite the reference cavity is exposed to the sample fluid providing an external pressure. Accordingly, the silicon diaphragm flexes responsive to the difference between the external pressure and the reference pressure.
The plates of the capacitive transducer of Clark, however, may be subject to shorting in an overpressure condition. An overpressure short may be addressed by providing an insulating layer or overpressure stop. See, for example, Cho, Najafi and Wise, "Secondary Sensitivities and Stability of Ultrasensitive Silicon Pressure Sensors," IEEE, 1990. The overpressure stop of Cho is a layer of an insulating glass on the face of the silicon diaphragm adjacent the reference cavity. Because the two adjacent layers are not homogeneous, however, flexing may generate internal stresses in the materials of the diaphragm. The inhomogeneity also results in a thermal expansion mismatch between the materials.
Both of the above mentioned capacitive transducers are also limited by the material characteristics of silicon which has a relatively low energy bandgap of 1.12 eV and other shortcomings, particularly for high temperature applications. Diamond, in contrast, is a preferred material for many microelectronic devices because it has semiconductor properties that are better than silicon, germanium or gallium arsenide. Diamond also provides a higher energy bandgap, a higher breakdown voltage and a higher saturation velocity than these traditional semiconductor materials.
These properties of diamond yield a substantial increase in projected cutoff frequency and maximum operating voltage compared to devices fabricated using silicon, germanium or gallium arsenide. Silicon is typically not used at temperatures higher than about 200.degree. C. and gallium arsenide is not typically used above 300.degree. C. These temperature limitations are caused, in part, because of the relatively small energy bandgaps for silicon (1.12 eV at ambient temperature) and gallium arsenide (1.42 eV at ambient temperature). Diamond, in contrast, has a large bandgap of 5.47 eV at ambient temperature, and is thermally stable up to about 1200.degree. C.
Diamond has the highest thermal conductivity of any solid at room temperature and exhibits good thermal conductivity over a wide temperature range. The high thermal conductivity of diamond may be advantageously used to remove waste heat. In addition, diamond has a smaller neutron cross-section which reduces its degradation in radioactive environments, i.e., diamond is a "radiation-hard" material. In addition, diamond has a higher gauge factor than conventional semiconductors.
Because of the advantages of diamond as a material for microelectronic devices, there is an interest in the growth and use of diamond for microelectronic transducers. A piezoresistive diamond pressure sensor is disclosed in Japanese Patent No. 03-063538 to Uesugi. This pressure sensor includes a piezoresistive diamond diaphragm, and a resistance detecting circuit located on the substrate rather than the diamond film. Accordingly, the signal processing circuitry fails to take advantage of diamond.
A microelectronic piezoelectric sensor formed in high energy bandgap semiconductors is disclosed, for example, in U.S. Pat. No. 4,706,100 to Tufte. This patent discloses a piezoresistive pressure sensor with a diaphragm including a layer of a high energy bandgap semiconductor which is epitaxially grown on silicon. The patent discloses that suitable large bandgap semiconductors have a suitable energy band structure to exhibit large piezoresistive effects and which can be grown on silicon in a good quality heteroepitaxial layer. Disclosed examples include p-type GaAs (1.4 eV bandgap), p-type Al.sub.x Ga.sub.1-x As alloys, and n- or p-type AlAs which are lattice matched to GaAs and allow bandgaps up to 2.1 eV and .beta.-SiC which has a bandgap of 2.7 eV. The patent also discloses integration of read-out electronics into the large bandgap semiconductor film.
Conventional semiconductor capacitive transducers are limited in temperature range and the ability to withstand harsh environments. Because of the advantages of diamond as a material for microelectronic devices, there is an interest in the growth and use of diamond for microelectronic transducers and particularly for sensors. Diamond pressure sensors, which may have several of the advantages of diamond, have only been described including piezoelectric resistors formed in a diamond layer.